class Solution
{
    int dx[4] = { 0,0,1,-1 };
    int dy[4] = { 1,-1,0,0 };
    int m, n;
public:
    void solve(vector<vector<char>>& board)
    {
        m = board.size(), n = board[0].size();
        for (int j = 0; j < n; j++)
        {
            if (board[0][j] == 'O') bfs(board, 0, j);
            if (board[m - 1][j] == 'O') bfs(board, m - 1, j);
        }
        for (int i = 0; i < m; i++)
        {
            if (board[i][0] == 'O') bfs(board, i, 0);
            if (board[i][n - 1] == 'O') bfs(board, i, n - 1);
        }
        for (int i = 0; i < m; i++)
        {
            for (int j = 0; j < n; j++)
            {
                if (board[i][j] == 'O') board[i][j] = 'X';
                else if (board[i][j] == '.') board[i][j] = 'O';
            }
        }
    }
    void bfs(vector<vector<char>>& board, int i, int j)
    {
        queue<pair<int, int>> q;
        q.push({ i,j });
        board[i][j] = '.';
        while (q.size())
        {
            auto [a, b] = q.front();
            q.pop();
            for (int k = 0; k < 4; k++)
            {
                int x = a + dx[k];
                int y = b + dy[k];
                if (x >= 0 && x < m && y >= 0 && y < n && board[x][y] == 'O')
                {
                    q.push({ x,y });
                    board[x][y] = '.';

                }
            }
        }
    }
};